[marktangotango]

Note I specified 'batch'. One mainframe app I'm familiar with handled > 50% of mutual fund transactions in the US. Including 401k, this could be > 100 million transactons per day when follow on broker/dealer cascade transactions are accounted for. Each transaction required non trivial amounts of processing, reporting, and logging.

Also note that a 'transaction' in this sense is not a http request response. The way it's used in mainframe systems it's a business transaction which can include 100's of smaller 'transactions'.

Banks and other financial systems can't perform daily reconciliation until markets close and stock and fund prices are known. Hence these systems store everything up to process in a nightly window.